1. Field of the Invention
The present invention relates to a spread spectrum communication device, and more particularly to a spread spectrum communication device having the capability of removing a narrow-band interfering signal present in a frequency band of a spread signal.
2. Description of the Related Art
FIG. 12 illustrates an example of a narrow-band interference signal elimination circuit used in a conventional spread spectrum communication device.
As shown in FIG. 12, the narrow-band interference signal elimination circuit 100 comprises a switch 101, a band-rejection filter 102, a bandpass filter 103, a detector 104, a maximum level detection circuit 105, a switch control circuit 106, an input signal terminal 107, and an output signal terminal 108. The bandpass filter 103 comprises a plurality of filters BPF1, BPF2, . . . , BPFn. The input of each of these filters is connected to the input signal terminal 107, and the output of each of these filters is connected to the maximum level detection circuit 105 via the corresponding one of the detectors DFT1, DFT2, . . . , DFTn. The output of the maximum level detection circuit 105 is connected to the switch 101 via the switch control circuit 106. The input terminal of the switch 101 is connected to the input signal terminal 107. The switch 101 has a plurality of output terminals which are connected to the filters BEF1, BEF2, . . . , BEFn, respectively, of the band-rejection filter 102. The outputs of the filters BEF1, BEF2, . . . , BEFn are combined together and connected to the output signal terminal 108. The band-rejection filter 102, the bandpass filter 103, and the detector 104 are each include an equal number of elements.
In the narrow-band interference signal elimination circuit 100, the frequency bands of the respective filter elements of the band-rejection filter 102 and those of the respective filter elements of the bandpass filter 103 correspond to the frequency subbands used in the spread spectrum communication. When a signal is input to the input signal terminal 107, the input signal is applied to the switch 101 and also to the detector 104 via the bandpass filter 103. The detector 104 detects the signal level for each frequency band. The maximum level detection circuit 105 evaluates the signals output from the respective elements of the detector 104, and determines the frequency band having a maximum signal level. The maximum level detection circuit 105 operates the switch 101 via the switch control circuit 106 so that the input signal is coupled to a band-rejection filter element having a frequency band corresponding to that of the bandpass filter element regarded as having the maximum signal level and thus the input signal components except for the signal component corresponding to the frequency band of the selected band-rejection filter element are output via the output signal terminal 108. When a narrow-band interfering signal is present within a communication frequency band, the signal component having a frequency band including the frequency of the interfering signal is removed by the narrow-band interference signal elimination circuit 100 described above.
The narrow-band interference signal elimination circuit 100 described above has a problem that a great number of bandpass filter elements and band-rejection filter elements are required, and thus a large installation area for them is required. This causes an increase in cost. Another problem is that when there are two or more narrow-band interfering signals having different frequencies, only one narrow-band interfering signal having the highest signal level is removed.
Thus it is an object of the present invention to provide a small-sized low-cost spread spectrum communication device having a narrow-band interference signal elimination circuit capable of covering all spread signal frequency bands.